Respiratory syncytial virus (RSV) infection is the primary cause of respiratory infection in young children, causing >120,000 hospitalizations in the US annually. RSV vaccine development has been hampered by the history of a failed vaccine trial in the early 1960s in which vaccinated children were not protected against subsequent natural infection but rather experienced more severe disease. Subsequent experiments in animal models and human subjects suggest the occurrence of severe RSV disease correlates with the induction of virus-specific Th2 CD4+ T cells and eosinophil recruitment and degranulation. The putative attachment G glycoprotein of RSV and killed virus vaccines containing adjuvants such as alum are particularly effective in inducing such disease-enhancing immune responses. The purposes of these studies are 1) to define the precise components of RSV immunity that predispose for severe RSV disease upon subsequent exposure to the virus, 2) to determine how the structure of RSV contributes to disease, and 3) with this better understanding of RSV pathogenesis, to rationally design RSV vaccines that protect against infection without enhancing disease.